The increasing cost of natural gas necessitates the continuing development of alternative fuels. One example of such is low-calorie fuel gas, also referred to in the following as synthesis gas. In principle, synthesis gas can be produced from solid, liquid or gaseous starting materials. Coal gasification, biomass gasification and coke gasification should be cited as the principal processes used in the context of synthesis gas production from solid starting materials.
In view of the ever more stringent requirements in respect of nitrogen oxide emissions, premix combustion is becoming increasingly important also for the combustion of low-calorie gases.
Premix burners typically include a premix zone in which air and fuel are mixed before the mixture is conducted into a combustion chamber. There, the mixture is combusted, generating a hot gas under increased pressure in the process. Said hot gas is directed onward to the turbine. The most important consideration in connection with the operation of premix burners is to restrict the nitrogen oxide emissions to a minimum and to avoid a flame blowback.
Synthesis gas premix burners are characterized in that synthesis gases are used as fuel therein. Compared with the traditional turbine fuels of natural gas and crude oil, which essentially consist of hydrocarbon compounds, the combustible constituents of the synthesis gases are essentially carbon monoxide and hydrogen. Depending on the gasification method and the overall system concept, the calorific value of the synthesis gas is roughly 5 to 10 times less than that of natural gas.
Due to its low calorific value fuel gas must accordingly be introduced into the combustion chambers at high volumetric flow rates. As a consequence thereof significantly larger injection cross-sections are required for burning low-calorie fuels, such as synthesis gases for example, than in the case of conventional high-calorie fuel gases. In order to achieve low NOx values it is, however, necessary to burn synthesis gas in a premix mode of operation.
Apart from the stoichiometric combustion temperature of the synthesis gas, a significant determining factor in avoiding temperature peaks and consequently in minimizing thermal nitrogen oxide formation is the quality of the mixing between synthesis gas and combustion air at the flame front. A spatially good mix of combustion air and synthesis gas is particularly difficult on account of the high volumetric flow rates of requisite synthesis gas and the correspondingly large spatial extension of the mixing region. On the other hand, not least for reasons of environmental protection and corresponding statutory guidelines on pollutant emissions, the lowest possible production of nitrogen oxide is an important requirement for combustion, in particular for combustion in the gas turbine plant of a power station. The formation of nitrogen oxides increases exponentially quickly with the flame temperature of the combustion. An inhomogeneous mixture of fuel and air results in a specific distribution of the flame temperatures in the combustion zone. In accordance with the cited exponential relationship between nitrogen oxide formation and flame temperature, the maximum temperature of such a distribution determines to a significant extent the amount of undesirable nitrogen oxides formed.
The individual fuel jets must penetrate into the mass air flow to an adequate depth in order to ensure satisfactory mixing between fuel and air. Compared to high-calorie burner gases such as natural gas, however, correspondingly larger, free injection cross-sections are necessary. The consequence of this is that the fuel jets seriously interfere with the air flow, ultimately leading to a local separation of the air flow in the wake region of the fuel jets. The backflow regions forming within the burner are undesirable and to be avoided at all costs in particular for the combustion of highly reactive synthesis gas. In the extreme case said local backflow regions lead within the mixing zone of the burner to a flame blowback into the premix zone and consequently result in damage to the burner.
The high reactivity of synthesis gas, in particular when there is a high percentage of hydrogen, also increases the risk of a flame blowback.
Furthermore, the larger injection cross-sections that are necessary for the synthesis gas generally lead to poor premixing of air and synthesis gas, thereby resulting in precisely said undesirable high NOx values.
In addition, drops in pressure frequently occur during the injection as a result of the high volumetric flow rate.
The mixing of synthesis gas with air is accomplished for example by means of swirling elements, such as described e.g. in EP 1 645 807 A1, or by means of an injection of the gas transversely with respect to the air flow. However, these techniques lead to a significant undesirable drop in pressure and can create undesirable wake regions which result in flame blowback.